1. Field of the Invention
The present invention relates to electron emitters that include carbon nanotubes and the application thereof in gas discharge devices. More particularly, the present invention relates to cathodes of fluorescent lamps that include carbon nanotubes.
2. Description of the Related Art
A source of electrons is required in the operation of many devices such as vacuum microelectronic, cathodoluminescent, and gas discharge devices. One common gas discharge device is a fluorescent lamp. In a gas discharge device, the electrons emitted from a cathode migrate to an anode while ionizing a background gas and propagating the discharge. At the cathode of a gas discharge device, the electrons are conventionally generated by thermionic or secondary emission processes. The thermionic emission process employs a low work function material which is raised to a high temperature to liberate electrons into the surrounding environment. Secondary emission processes employ a cathode material having a high secondary electron yield coefficient to liberate electrons when it is impacted by ions, other electrons, or photons.
Thermionic emission cathode materials are heated to very high temperatures for their operation, typically greater than 1000° C., with the attendant shortcomings. First, this heating requires power, which reduces the overall energy efficiency of the gas discharge device. Second, the emissive material on the cathode slowly boils off into the surrounding environment at the required elevated temperature, resulting in limited lifetime of the cathode. This boiled-off material also can have deleterious effects on the performance of other chemically sensitive materials in the gas discharge device, such as phosphors. Finally, the elevated cathode temperature renders the cathode materials moderately chemically reactive, and therefore care must be taken in engineering the surrounding environment such that other materials in the system do not react with and poison the cathode emissive materials.
Cathodes designed to use secondary electron emission processes rely on incident ions, electrons, or photons to initiate a cascade which can then be self-supporting. To generate enough ions for self-supporting of a discharge, the cathode fall is usually much higher than that of the thermionic-emission devices. Electron emission from the cathode is primarily due to ion impact, which transfers a significant amount of energy to the cathode substrate as a result of the surface collision, resulting in a net energy loss from the device beyond the energy required to liberate the counter-propagating electrons. These factors result in a low efficiency in generating electrons. Furthermore, ion impact also results in sputtering of the cathode material and leads to some of the problems of thermionic emission.
These shortcomings are intrinsic to currently used cathodes in gas discharge devices. The energy efficiency of these devices would be greatly enhanced if the energy supplied to the cathode is used only for liberating electrons from its surface. Field emitters theoretically can avoid many of these shortcomings. Field emitters are devices that provide emission of electrons from the surface of an electrical conductor or semiconductor under an imposition of an electric field at temperatures not much higher than room temperature. Typically, the electric field required for electron emission from a surface of field emitters is on the order of about 108−109 V/m. However, this required high electric field is not available in typical gas discharge devices such as fluorescent lamps. Recently, carbon nanotubes have received much attention as a promising material for field emission applications because their diameters, in the nanometer range, approach molecular scale, which offer a great intensification of electric field at their tips. Research has been directed to using carbon nanotube arrays as electron guns for flat panel display. Other structures having a nano dimension also have been used for field emission. U.S. Pat. No. 5,495,143 discloses the field intensification with the assistance of nanostructures made of metals and metallic compounds having sharp tips was used to provide field emission in gas discharge devices. A similar concept disclosed in U.S. Pat. No. 5,686,789 was applied in cathodes of other gas discharge devices, which cathodes include microscopic holes of non-specific shapes. However, there has not been any attempt to use carbon nanotubes in the area of gas discharge devices to improve the energy efficiency thereof. In light of the inefficiencies of gas discharge devices, it is very desirable to provide a cathode material that can be used for an electron emitter in these devices and does not require as high an electric field as prior-art materials for field emission. It is also desirable to provide an electron emitting material that can be made easily and inexpensively. It is further desirable to use carbon nanotubes to improve the energy efficiency of gas discharge devices.